All true
flutes work basically the same. The blown air directed
from the lips is divided and oscillates as it strikes
the far edge of the blow hole. The molecules of
the air column within the flute’s tube are then
set into a vigorous back and forth motion sounding
a fundamental note.

In transverse
flutes this occurs when blown across a hole cut
in the side wall of the tube. One end of the transverse
flute’s tube is stopped (closed) creating a
virtual baffle. Also the inside tube wall
immediately below the blow hole provides baffling
critical to its function. A baffle holds air,
like the bottle in figure one. When
blowing into a baffled structure whether a bottle
or a transverse flute, the jet stream and resulting
note will be stable as expressed by straight
lines originating from the lips in figure one and figure
two.

In vertical
flutes, more commonly known as the end blown flute,
sound begins the same as air is blown across one
end of an open tube as illustrated in figure three. However, the vertical flute
is essentially unbaffled as its tube is open throughout.
This unbaffled acoustical environment allows the
jet stream and resultant note to become intermittenly
unstable as expressed by tear dropped shaped lines
in figure three.

Pictured
here is a drawing excerpted from a vertical flute
patent issued in the late 19th century. Even
though its headjoint is stopped at both ends
“a-a”, its blow hole is located in the same
position “c” (unbaffled) as an end blown flute
and would exhibit jet stream instability if
actually built.